#include <bits/stdc++.h>
using namespace std;

double xa, ya, xb, yb;
int main() {
    cin >> xa >> ya >> xb >> yb;
    double xl = abs(xa - xb);
    double yl = abs(ya - yb);
    double len = sqrt(xl * xl + yl * yl);
    cout << fixed << setprecision(3) << len;
    return 0;
}
